1. Field of the Invention
The present invention relates to a Mn—Zn based ferrite, a magnetic core for a transformer and a transformer, and particularly relates to a Mn—Zn based ferrite used in a magnetic core of a communication transformer, etc. and having excellent characteristics of direct current pre-magnetization in a wide temperature range, a magnetic core for a transformer and a transformer.
2. Description of the Related Art
Since a Mn—Zn based ferrite shows a little deterioration of initial permeability in a high frequency band and an increase of a power lose when used as a power source transformer, it is a main magnetic material as a communication transmission ferrite and a switching power ferrite. Along with a use environment becoming diversified due to more compact and more mobile electronic devices, there has been a demand of making temperature dependency of communication electric circuit characteristics using them small.
In response to the demand, there has been a proposal for improving magnetic characteristics of a M—Zn based ferrite in a wide temperature range as explained below.
For example, the Japanese Unexamined Patent Publication No. 59-50072 discloses a ferrite material having an improved permeability and temperature coefficient of a magnetic lose, obtained by including CaO, CoO, ZrO2 or CaO, CoO and V2O5 as other subcomponents in a Mn—Zn based ferrite and controlling an oxygen concentration in a cooling atmosphere after sintering.
Also, the Japanese Unexamined Patent Publication No. 11-302069 discloses a Mn—Zn based ferrite having an initial permeability of 10000 or more at −20° C. to 100° C., by including CaO, SiO2 and CoO as subcomponents in the Mn—Zn based ferrite and controlling the composition.
Recently, however, further lighter, thinner and more compact electronic devices are demanded and excellent characteristics of direct current pre-magnetization (inductance characteristics under direct current bias superimposition) in a wide temperature range and a high permeability have been demanded due to a higher density and higher frequency in circuit d signing.
However, in the ferrites described in the above patent articles, the magnetic characteristics under direct current bias superimposition in a wide temperature range are insufficient.